Inositol (myo-inositol)

What it is

Inositol is a sugar-like molecule your body makes itself and also gets from foods such as fruit, beans, grains and nuts. It is sometimes called vitamin B8, but it is not a true vitamin because the body can make it.

Supplements are mostly myo-inositol, and sometimes combined with D-chiro-inositol. It is best known for use in polycystic ovary syndrome (PCOS), where small trials have looked at insulin and ovulation, and it is also studied for mood and anxiety.

How it works

Inositols act as secondary messengers inside cells, relaying signals from insulin and some neurotransmitter receptors. In PCOS, where insulin signalling is often disrupted, supplementing is thought to improve insulin sensitivity. In the brain, it may influence serotonin signalling.

What the evidence says

• Moderate Insulin sensitivity in PCOS
  Several small trials and reviews report improved insulin markers, but trial quality varies.
• Moderate Ovulation and cycle regularity in PCOS
  Some trials show improved ovulation rates; results are not uniform and it is not a replacement for medical care.
• Limited Gestational diabetes risk
  Some trials in pregnancy suggest lower risk; this should only be considered with a midwife or doctor.
• Limited Anxiety, panic and depression
  Small older trials used high doses; evidence is weak and mixed.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• For PCOS, studies often use about 4 g a day of myo-inositol, commonly 2 g twice daily, sometimes with a 40:1 ratio of myo to D-chiro inositol.
• Mood studies used much higher doses (12 g or more), which are more likely to cause stomach side effects.
• Can be taken with or without food; powder mixes into water.
• Anyone with PCOS or trying to conceive should discuss it with their GP or specialist first.

Forms and what to look for

• Myo-inositol alone is the most studied form.
• Myo plus D-chiro in a 40:1 ratio mimics the body's natural plasma ratio and is common in fertility products.
• Powder is the most cost-effective for multi-gram doses.
• Avoid products using mostly D-chiro-inositol at high doses, which may affect ovarian function.

What to expect, and when

• Changes in cycles or insulin markers in studies appeared over 3 to 6 months.
• Mood effects, if any, took several weeks.
• Many people see no obvious change.

Side effects

• Nausea, gas, bloating or diarrhoea, especially at higher doses.
• Headache or dizziness.
• Fatigue or insomnia in some people.
• Rarely, a worsening of mood in people with bipolar disorder.

Interactions

• Lithium; inositol may interact with lithium-related pathways, so ask a clinician.
• Diabetes medicines such as metformin or insulin; it may lower blood sugar further.
• Antidepressants and other mood medicines; limited data.
• Fertility treatments; discuss with your clinic.

Who should avoid it

• People with bipolar disorder unless supervised by a clinician.
• Pregnancy and breastfeeding unless advised by a doctor or midwife.
• People on diabetes medicines without monitoring.
• Anyone with kidney problems should check first.

Risks and safety

• High mood-study doses are not suitable for self-treatment.
• PCOS has multiple causes and needs a proper diagnosis and management plan.
• Not on the WADA prohibited list to our knowledge.
• Long-term high-dose data is limited.

Buying well: quality and testing

• Check that the label specifies myo-inositol and the grams per serving.
• Pure powders are easy to verify; capsule products can contain very little.
• Prefer third-party tested brands.
• Check ratios if buying a myo/D-chiro blend.

Bottom line

Inositol is one of the more plausible supplements for PCOS-related insulin and cycle issues, with effects that are real but moderate and not a cure. It suits people who have discussed it with a clinician. For anxiety or depression, evidence is too thin to rely on it over established treatments.